Add onFinishing callback about kotter

[bitspittle]

This may already be done, or at least trivial, using KonsoleData. However, that's not currently available to the RunScope yet, and maybe we shouldn't expose it directly. We'll need to think about the API carefully.

But otherwise, right now you'd do this as something like

class BlinkingCursorState {
   object Key = KonsoleData.Key<BlinkingCursorState>()
   var blinkOn by KonsoleVar(false)
}

/* ... */

data.putIfAbsent(
   BlinkingCursorState.Key,
   provideInitialValue = { BlinkingCursorState() }
   dispose = { blinkOn = false }
)

It's possible that

.run {
   onFinishing {
      /* ... */
   }
}

works under the hood by putting in a dummy object into the block data which triggers the callback on dispose.